Hermetic sealing of glass substrates to create a vacuum or inert gas environment therebetween is typically made possible using barriers of usually glassy or metallic (e.g., eutectic) materials that are impermeable to ingress of gasses over a long time period, typically many orders of magnitude longer than the device operating lifetime. As will be understood, permeability involves two steps. These steps include dissolution and diffusion. Heremetic sealing keeps, for example, water, other liquids, oxygen and other gaseous contaminant molecules out of packages that hold, for example, and without limitation, a vacuum (e.g., VIG window units, thermos flask, MEMS, and the like), or sensitive materials, such as, for example, and without limitation, organic emitting layers (e.g., used in OLED devices), semiconductor chips, sensors, optical components, or the like, held in an inert atmosphere. Gas tight packaging of the complex interiors of such assemblies has posed obstacles in the later stages of processing of such packages, such as, for example prior to pumping and tip fusing in the case of VIG window units, or last processing steps in the manufacture of OLED devices.
Some example VIG configurations are disclosed, for example, in U.S. Pat. Nos. 5,657,607, 5,664,395, 5,657,607, 5,902,652, 6,506,472 and 6,383,580, the disclosures of which are all hereby incorporated by reference herein in their entireties.
FIGS. 1 and 2 illustrate a typical VIG window unit 1 and elements that form the VIG window unit 1. For example, VIG unit 1 may include two spaced apart substantially parallel glass substrates 2, 3, which enclose an evacuated low-pressure space/cavity 6 therebetween. Glass sheets or substrates 2,3 are interconnected by a peripheral edge seal 4 which may be made of fused solder glass or the like, for example. An array of support pillars/spacers 5 may be included between the glass substrates 2, 3 to maintain the spacing of substrates 2, 3 of the VIG unit 1 in view of the low-pressure space/gap 6 present between the substrates 2, 3.
A pump-out tube 8 may be hermetically sealed by, for example, solder glass 9 or the like to an aperture/hole 10 that passes from an interior surface of one of the glass substrates 2 to the bottom of an optional recess 11 in the exterior surface of the glass substrate 2, or optionally to the exterior surface of the glass substrate 2. A vacuum is attached to pump-out tube 8 to evacuate the interior cavity 6 to a low pressure that is less than atmospheric pressure, for example, using a sequential pump down operation. After evacuation of the cavity 6, a portion (e.g., the tip) of the tube 8 is melted to seal the vacuum in low pressure cavity/space 6. The optional recess 11 may retain the sealed pump-out tube 8. Optionally, a chemical getter 12 may be included within a recess 13 that is disposed in an interior face of one of the glass substrates, e.g., glass substrate 2. The chemical getter 12 may be used to absorb or bind with certain residual impurities that may remain after the cavity 6 is evacuated and sealed.
VIG units with peripheral hermetic edge seals 4 (e.g., solder glass) are typically manufactured by depositing glass frit or other suitable material in a solution (e.g., frit paste) around the periphery of substrate 2 (or on substrate 3). This glass frit paste ultimately forms the edge seal 4. The other substrate (e.g., 3) is brought down on substrate 2 so as to sandwich spacers/pillars 5 and the glass frit solution between the two substrates 2, 3. The entire assembly including the glass substrates 2, 3, the spacers/pillars 5 and the seal material (e.g., glass frit in solution or paste), is then typically heated to a temperature of at least about 500° C., at which point the glass frit melts, wets the surfaces of the glass substrates 2, 3, and ultimately forms a hermetic peripheral/edge seal 4.
After formation of the edge seal 4 between the substrates, a vacuum is drawn via the pump-out tube 8 to form low pressure space/cavity 6 between the substrates 2, 3. The pressure in space 6 may be produced by way of an evacuation process to a level below atmospheric pressure, e.g., below about 10−2 Torr. To maintain the low pressure in the space/cavity 6, substrates 2, 3 are hermetically sealed via the edge seal and sealing off of the pump-out tube. Small high strength spacers/pillars 5 are provided between the transparent glass substrates to maintain separation of the approximately parallel glass substrates against atmospheric pressure. As noted above, once the space 6 between substrates 2, 3 is evacuated, the pump-out tube 8 may be sealed, for example, by melting its tip using a laser or the like.
High-temperature bonding techniques such as, for example, anodic bonding and glass frit bonding, as discussed above, have been widely used method for hermetically sealing (e.g., forming an edge seal) components made of silicon, ceramics, glass, or the like. The heat required for these high-temperature processes is typically in the range of about 300° C. to 600° C. These conventional bonding techniques typically require oven-intensive bulk heating in which the entire device (including the glass and any components housed within the glass housing) comes to near thermal equilibrium with the oven for a seal to form. As a result, a relatively long period of time is required to achieve an acceptable seal. For example, as the device size L increases, the sealing time may typically increase on the order of L3. It is also the case that the most temperature sensitive component determines the maximum allowable temperature of the entire system. Thus, high-temperature sealing processes discussed above (e.g., anodic bonding and glass frit bonding) are not suitable for fabricating heat-sensitive components such as, for example, tempered VIG units and encapsulating sensitive components, such as, for example, OLED devices. In the case of tempered VIG units, the thermally tempered glass substrates of a VIG unit would rapidly lose temper strength in the high-temperature environment. In the case of an example OLED package, certain functional organic layers would be destroyed at temperatures of 3000-600 degrees C. (sometimes even as low as 100° C.). In the past, one way to address this with high-temperature bulk sealing processes was to develop lower temperature frits, while still using bulk thermal equilibrium heating processes.
By way of background, glass frits and/or solders are typically mixtures of glass material and metallic oxides. Glass composition may be tailored to match the coefficient of thermal expansion (CTE) of the bonding substrates. Lead-based glasses are the most common bonding/sealing material/technique used commercially in cathode ray tubes (CRT), plasma displays and VIG window units. Lead-based glass frits are also among the least permeable glass sealing materials. Traditionally, these solders are based on glassy materials and de-vitrification is suppressed.
Glass frits or solders are typically made up of a base glass, a refractory filler and a vehicle. The base glass forms the bulk of the frit or solder. The filler reduces the CTE as well as matching it to the glass substrates to be joined. This matching enhances the mechanical strength, reduces interfacial stress and improves the crack resistance of the seal. The vehicle is typically made of a solvent that provides fluidity for screen printing and an organic binder.
Among the advantages of these types of glass frits or solders is that they include a relatively low melting point (e.g., in a range of about 480° C.-520° C.) glass that will stick to most semiconductor materials, including, but not limited to, glass, silicon, silicon oxide, most metals and ceramics, making bonding techniques using these types of materials versatile and widely accepted.
There are many different types of commercially available glass fit materials having various melting points, CTEs, organing binders and screen printing properties. However, almost all lower melting point formulations of glass fit or solder contain some lead. This may potentially become a major drawback, as the EU and Japan are severely limiting, if not forbidding, the use of lead in electronics manufacturing in the coming years. In the last few years, frits or solders based on bismuth oxides have had some success in replacing lead based frits, however the melting temperature (Tg) of these types of frits is still typically above about 450° C. As with lead based fits, these bismuth oxide based frits cannot be used for fabrication of temperature sensitive devices using conventional oven bulk heating processes. Lower Tg (e.g., 375° C.-390° C.) fits based on vanadium barium zinc oxides (VBZ) have also been developed, including, but not limited to, VBaZn, V phosphates, SnZnPO4. However, widespread usage of these types of frits has heretofore been limited.
Therefore, there exists a need for a seal processing technique that does not involve heating the entire article to be sealed to high temperature(s). In other words, a technique that utilizes localized heating substantially limited to an area in which the material used to form the seal, for example, frit-inclusive seal material, is disposed is needed. Such a localized approach may be achieved, for example, and without limitation, via controlled laser heating of the seal material and resulting vitrified solder bead. In such a technique, the localized heating may be limited to the frit, or an area proximate where the frit is deposited, and the heating profile may be kept moderate. It may also be advantageous to include a system for localized heat sinking to control lateral heat flow to reduce cracking due to stress and the relatively large differences in expansion characteristics and temperature gradients. The molten seam or bead formed using this type of localized process is used to join the desired parts, e.g., the glass substrates of a VIG unit. This process may be referred to as laser stitching since the molten seam or bead in effect stitches the joining parts together. According to certain alternative example embodiments, such localized controlled laser heating may also be used with eutectic sealing materials.
In conventional oven bulk heating processes, a frit paste is typically applied to a glass substrate and, in a process referred to as drying, is heated at a relatively low temperature (e.g., in a range of about 120° C.-150° C.) to remove the solvent from the vehicle. Next, the glass is heated to a higher temperature in a glazing process to glaze the frit and drive out the organic binder material. In a subsequent vitrification process frit is then vitrified by raising its temperature to its melting point to form a continuous film comprising the glass network. Finally, the glass substrates are aligned and heated above the glass melting temperature while the substrates are squeezed together to form the final hermetic seal between the substrates.
According to an example, non-limiting embodiment, after the frit paste is applied, dried and glazed, localized vitrification (e.g., instead of bulk oven processing) is performed using a laser in air or an inert atmosphere. During this process, it may be preferable to include localized heat sinking to control lateral heat flow. A roller mechanism or application of a vacuum pump-down may optionally be used to apply force between the substrates being joined together. The substrates are aligned and the frit sealing material is brought to temperature(s) above its melting temperature by localized laser irradiation to perform laser induced bonding of the frit and/or solder to the glass substrate(s). In a subsequent step, laser annealing of the solder may also be performed to reduce the stress between the seal and the substrate(s). It has also been found that roughening of the glass prior to the application of the frit sealing material provides improved bonding strength of the frit to glass interface.
According to another example, non-limiting embodiment, the localized heating approach may be achieved via controlled laser heating of the vitrified solder bead and/or seam. The amount of power delivered to the frit may be controlled by a feedback loop using the temperature at some distance from the thermal spike as input to a feedback loop. Monitoring and controlling the high process temperatures in the joining area via feedback control of the laser power and duration provides advantages in managing the stress and breakage of the device.
According to certain alternative example embodiments, a eutectic sealing material (e.g., metallic or metal alloy solder) may be used instead of glass based or glass inclusive frit sealing materials. In such alternative example embodiments, because metal cannot typically be used directly on glass, an absorber layer may be interposed between the eutectic seal material and the glass substrate(s). The absorber film/layer may, for example, and without limitation, include a first layer of or including silicon nitride and a second layer of or including a metal such as electrodeless Ni (or a metal alloy). In such example embodiments, the energy of the laser beam is transmitted through a glass substrate and absorbed by the absorber film. As a result, for example, the Si and/or metal of the absorber locally heats up, melts the eutectic material, and the Si is bonded to the glass substrate(s). As noted above, control of the laser may be achieved using a feedback loop in which the high process temperatures in the joining area are monitored and fed back to control the laser power and duration.
In certain example embodiments of this invention, there is provided a method of making a vacuum insulated glass (VIG) window unit, the method comprising: providing a first substrate; applying a seal material to an area of the first substrate to be sealed; forming a seal by at least (a) vitrifying the seal material using laser irradiation, the laser irradiation exposing the seal material but not being directed toward a majority of the first substrate, and (b) bonding the first substrate to a second substrate by continuing to irradiate the vitrified seal material with laser irradiation to melt the seal material; and evacuating a cavity formed between the first and second substrates and defined by the seal to a pressure lower than atmospheric pressure.
In certain example embodiments, there is provided a method of making a bonded article having a cavity formed between two glass substrates, comprising: providing first and second glass substrates; applying a sealing material to one of the glass substrates; and forming a seal by irradiating the sealing material with a laser, wherein said cavity is defined by geometries of the glass substrates and the seal, wherein the cavity is either reduced to a pressure below atmospheric pressure or provided with an inert gas atmosphere and may have disposed therein temperature sensitive components.
In certain embodiments of this invention, there is provided a vacuum insulated glass (VIG) window unit, comprising: first and second substantially parallel spaced apart substrates comprising glass that are bonded together by an edge seal, said first and second substrates and said edge seal defining a cavity having a pressure lower than atmospheric pressure; and wherein said edge seal comprises (i) a metallic or substantially metallic layer; and (ii) an absorber film. The edge seal may be formed via at least laser irradiation.
These and other embodiments and advantages are described herein with respect to certain example embodiments and with reference to the following drawings in which like reference numerals refer to like elements, and wherein: